RP Photonics

Encyclopedia … combined with a great Buyer's Guide!


Glossary and List of Formula Symbols

On this page, you first find the glossary of photonics terms, which is based on the definitions in the encyclopedia articles. Further below there is a list of common formular symbols.

Glossary of Photonics Terms

This page summarizes the definitions of photonics terms, as they appear below the headings of many of the encyclopedia pages.


Abbe number: a measure for the chromatic dispersion of a transparent material

ABCD matrix: a 2-by-2 matrix describing the effect of an optical element on a laser beam

absorbance: the logarithm with base 10 of the inverse transmittance

absorptance: the fraction of incoming radiant flux which is absorbed by an object

absorption: a process where light energy is converted to another form of energy

absorption coefficient: a logarithmic measure for the distributed absorption in a medium

absorption length: the length of which absorption reduces the optical intensity to 1/e of its initial value

acceptance angle in fiber optics: the maximum incidence angle of a light ray which can be used for injecting light into a fiber core or waveguide

achromatic optics: optical devices or setups with minimized chromatic aberrations

acousto-optic deflectors: acousto-optic devices for deflecting and scanning laser beams

acousto-optic frequency shifters: acousto-optic devices for shifting the optical frequency of a laser beam

acousto-optic modulators: optical modulators based on the acousto-optic effect

acousto-optic Q switches: acousto-optic modulators used for Q switching of lasers

acousto-optic tunable filters: acousto-optic devices which can be used for filtering light, controlled with an RF input

active mode locking: a technique of mode locking, based on active modulation of the intracavity losses or the round-trip phase change

additive-pulse mode locking: a technique for mode locking a laser, using a nonlinear interaction in an external resonator

adiabatic soliton compression: a pulse compression technique based on the adaptation of solitons to slowly varying propagation parameters

afocal optical systems: optical systems which output parallel rays for parallel input rays

alexandrite lasers: solid-state lasers based on alexandrite (Cr3+:BeAl2O4) crystals

alignment lasers: lasers used for alignment purposes

alignment sensitivity of optical resonators: the sensitivity of a laser, an optical resonator or some other device with respect to misalignment

all-solid-state lasers: laser systems containing solid-state devices only, in particular no discharge lamps or gas or dye lasers

amplification factor: the factor by which the power of a signal is amplified

amplified spontaneous emission: a process where spontaneously emitted radiation (luminescence) is amplified

amplifier chains: amplifiers consisting of several stages

amplifier noise: noise introduced to a signal in an amplifier device

amplifiers: devices for amplifying the power of light beams

amplitude-squeezed light: light with an intensity noise below the shot noise level

anamorphic prism pairs: prisms pairs for reshaping laser beams

anti-reflection coatings: optical thin-film coatings for reducing reflections from surfaces

aperture stops: apertures which limit the angular range of received input light of imaging systems

argon ion lasers: gas lasers based on light amplification in ionized argon in a gas discharge

arrayed waveguide gratings: optical filter or multiplexer devices based on arrays of waveguides

aspheric optics: optical elements with non-spherical surfaces

astigmatism: a kind of optical aberrations; alternatively, a defect of optical lenses or a property of laser beams

autocorrelators: devices for measuring the intensity or field autocorrelation function of light, mostly used for determining the duration of ultrashort pulses

avalanche photodiodes: photodiodes with internal signal amplification through an avalanche process


B integral: a measure of the nonlinear phase shift of light, e.g. in an amplifier

balanced photodetection: a method of photodetection which is sensitive to differences in optical powers but not to common noise

ball lenses: lenses which have the geometric form of a sphere

band gap: a range of energies for which there are no states of carriers in a medium

bandwidth: the width of some frequency or wavelength range

bandwidth–distance product: product of length and maximum signal bandwidth of a fiber-optic link

bandwidth-limited pulses: pulses with a duration as short as possible with their optical spectrum

beam collimators: devices for collimating optical beams

beam combining: a class of techniques for power scaling of laser sources by combining the outputs of multiple devices

beam divergence: a measure for how fast a laser beam expands far from its focus

beam dumps: devices for blocking and absorbing light beams

beam expanders: optical devices for modifying the beam radius of a collimated beam

beam homogenizers: devices for producing beam profiles with homogeneous intensity distribution

beam parameter product: product of the beam radius in a focus and the far-field beam divergence

beam pointing fluctuations: fluctuations of the propagation direction of a laser beam

beam profilers: devices for measuring the intensity profile of a laser beam

beam quality: a measure for how well a laser beam can be focused

beam radius: a measure of the transverse extension of a light beam

beam shapers: optical devices for modifying the shapes of laser beams

beam shutters: devices for blocking or releasing light beams

beam splitters: devices for splitting a laser beam into two or more beams

beam waist: location with minimum beam radius

beat note: an oscillation of the optical intensity arising from the superposition of light with different optical frequencies

Beer–Lambert law: a relation for the dependence of absorption coefficients on concentrations

bend losses: propagation losses in an optical fiber (or other waveguide) caused by bending

birefringence: the polarization dependence of the refractive index of a medium

birefringent phase matching: a technique of phase matching based on the birefringence of a crystal material

birefringent tuners: devices for wavelength tuning of lasers, based on birefringence and polarization rotation

bit error rate: average fraction of wrongly transmitted bits in a communication link

black coatings: coatings with strong light absorption

blue lasers: lasers emitting blue light

Bragg gratings: reflecting structures with a periodic refractive index modulation

Bragg mirrors: mirror structures based on Bragg reflection at a period structure

Brewster plates: coplanar plates inserted into beams at Brewster's angle

Brewster windows: transparent plates which are oriented at Brewster's angle such that parasitic reflection losses are minimized.

Brewster's angle: an angle of incidence at which there is no reflection of p-polarized light at an uncoated optical surface

brightness: a term mostly used in a qualitative way, related to the output power and beam quality of a laser; quantitatively: often used instead of radiance or luminance

brightness converters: optically pumped devices where the generated optical output has a higher brightness than the optical pump source

Brillouin scattering: a nonlinear scattering effect involving acoustic phonons

broad area laser diodes: laser diodes with a strongly asymmetric shape of the emitting region

bulk lasers: lasers based on bulk crystals as gain media


cameras: optical instruments for recording still or moving images

cardinal points: a set of points which characterize the function of an optical element or a group of elements in Gaussian optics

carrier–envelope offset: the offset between the optical phase and the maximum of the wave envelope of an optical pulse

causality: the concept of relating effects to causes, or the compatibility of phenomena with this concept

cavities: resonators for light (or for microwaves)

cavity dumping: a method for extracting intense short or ultrashort pulses from a laser, using an optical switch in the laser resonator

ceramic gain media: laser gain media which have a ceramic (polycrystalline) microscopic structure

chemiluminescence: luminescence arising from chemical reactions

chirp: time dependence of the instantaneous frequency of an optical pulse

chirped mirrors: Bragg-type dispersive mirrors with a spatial variation of the Bragg wavelength

chirped-pulse amplification: a technique for amplifying pulses to very high optical intensities while avoiding excessive nonlinear pulse distortions or optical damage

chromatic aberrations: image distortions caused by wavelength-dependent optical effects

chromatic dispersion: the frequency dependence of the phase velocity in a transparent medium

chromium-doped gain media: laser gain media doped with chromium ions

cladding mode strippers: devices which can remove light from a fiber cladding

cladding modes: modes in optical fibers (or other waveguides) which are not restricted to the region around the core

cleaning of optics: cleaning of optical elements

cleaving of fibers: preparing fiber ends with clean optical surfaces by controlled breaking

clustering: the tendency e.g. of laser-active ions in laser gain media to form clusters in their host medium

CO2 lasers: infrared lasers based on a gas mixture in which light is amplified by carbon dioxide molecules

coherence: a fixed phase relationship between the electric field values at different locations or at different times

coherence length: a measure of temporal coherence, expressed as the propagation distance over which the coherence significantly decays

coherence time: a measure of temporal coherence, expressed as the time over which the field correlation decays

coherent beam combining: a class of methods for beam combining, requiring mutual coherence of the combined beams

coherent states: a special kind of pure quantum-mechanical states of light

cold mirrors: mirrors which can reduce the heat load in an optical system by reflecting only visible radiation

collimated beams: laser beams with weak divergence

composite laser crystals: laser crystals consisting of several parts of different materials or with different chemical compositions (e.g. doping concentrations)

condensers: lenses or lens systems for conditioning light from an illumination source

confocal scanning microscopes: optical microscopes with enhanced depth resolution based on the confocal measurement principle

conjugate planes: pairs of planes where an optical system images one into the other and vice versa

continuous-wave operation: operation mode of a laser with continuous light emission

cooperative lasing: the phenomenon that laser action at one wavelength can facilitate lasing at another wavelength

core-less end caps: end pieces attached to fibers, containing no fiber core

corner cube prisms: prisms with three reflecting surfaces which act as retroreflectors

critical phase matching: phase matching of a nonlinear interaction by adjustment of a propagation direction

cross-phase modulation: a nonlinear effect where the optical intensity of one beam influences the phase change of another beam

cryogenic lasers: lasers where the gain medium is operated at cryogenic temperatures

crystal ovens: temperature-controlled ovens for keeping optical crystals at the desired operation temperature

crystalline mirrors: multilayer mirrors where the reflecting parts consist of a monocrystalline material

custom optics: optical elements made according to customer-specific specifications

cut-off wavelength: a wavelength above which a guided mode of a waveguide ceases to exist

cylindrical lenses: lenses which focus or defocus light only in one direction


dark current: a current from a photodetector which occurs even in the absence of a light input

decibel: a logarithmic measure for power ratios, applied e.g. to optical powers or to noise powers

depolarization loss: losses of optical power in a laser resonator, caused by depolarization e.g. in a laser crystal

depth of field: the distance between the nearest and furthest objects that can be imaged with reasonably sharp focus for a given focus setting

depth of focus: the half width of the range of longitudinal positions in which a reasonable focus for a film or image sensor is achieved

detectivity: the inverse of the noise equivalent power

diameter–divergence product: product of the diameter (at a beam waist) and the divergence angle of a laser beam

diaphragms: circular (or approximately circular) openings through which light can travel

dichroic mirrors: mirrors with significantly different reflection or transmission properties at two different wavelengths

dielectric coatings: thin-film coatings made of transparent dielectric materials, e.g. for laser mirrors or anti-reflection coatings

dielectric mirrors: mirrors consisting of multiple thin layers of different transparent optical materials

differential mode delay: the range of time delay values for signals in a telecom fiber

diffraction: wave phenomena which occur when light waves hit some structure with variable transmission or phase changes

diffraction gratings: optical components containing a periodic structure which diffracts light

diffraction-limited beams: beams with a minimum possible beam divergence for a given waist radius

diffractive optics: optical elements with operation principles based on diffraction of light

diode bars: a type of semiconductor laser containing a one-dimensional array of broad-area emitters

diode lasers: semiconductor lasers based on laser diodes

diode-pumped lasers: solid-state lasers which are pumped with laser diodes

diode stacks: arrangements of multiple diode bars, delivering very high output power

dioptric power: the inverse of the focal length

direct diode lasers: diode lasers which are directly used e.g. for material processing

dispersion: the dependence of the phase velocity in a medium on the optical frequency or the propagation mode

dispersion compensation: the control of the overall chromatic dispersion of a system by adding optical elements with a suitable amount of dispersion

dispersion compensation modules: modules used for dispersion compensation

dispersion-decreasing fibers: optical fibers where the chromatic dispersion changes monotonously along the propagation direction

dispersion management: the use of tailored chromatic dispersion to enhance the function of optical devices or systems

dispersion-shifted fibers: fibers with a non-standard zero dispersion wavelength

dispersive mirrors: mirrors which provide some amount of chromatic dispersion for the reflected beam

dispersive wave: a linearly propagating wave which is split off by a soliton wave under certain conditions

distance measurements with lasers: measurements of distances using laser light

distributed amplifiers: fiber amplifiers in fiber-optic data links, where the amplification occurs within a large length of transmission fiber

distributed Bragg reflector lasers: lasers containing distributed Bragg reflectors as end mirrors

distributed feedback lasers: lasers where the whole laser resonator consists of a periodic structure, in which Bragg reflection occurs

divided-pulse amplification: a method of amplifying intense ultrashort pulse while avoiding excessive nonlinear effects

doped insulator lasers: lasers with a solid-state gain medium containing a laser-active dopant

doping concentration: the concentration of some dopant, e.g. of laser-active ions in a laser gain medium

Doppler broadening: broadening of the linewidth of atomic transitions, caused by random motion of the emitting atoms

Doppler cooling: a technique of laser cooling based on velocity-dependent absorption processes

Doppler limit: a limit for the temperature which is achievable with certain laser cooling techniques

double-clad fibers: optical fibers with different waveguide structures for pump and signal light

double pulses: two closely spaced optical pulses

dye lasers: lasers based on liquid or solid dyes as gain media


edge-emitting semiconductor lasers: semiconductor lasers emitting light along the plane of the wafer

effective mode area: a quantitative measure of the area which a waveguide or fiber mode effectively covers in the transverse dimensions

effective nonlinear coefficient: a coefficient for quantifying the strength of a nonlinear interaction

effective refractive index: a number quantifying the phase delay per unit length in a waveguide, relative to the phase delay in vacuum

effective transition cross sections: a modified type of transition cross sections which apply to optical transitions between Stark level manifolds

electro-optic effect: the phenomenon that the refractive index of a material can be modified with an electric field

electro-optic modulators: optical modulators based on the electro-optic effect

electro-optic sampling: an optical sampling technique based on the electro-optic effect

electroabsorption modulators: optical modulators based on the Franz–Keldysh effect

electroluminescence: luminescence arising from electrical influences

electronic dispersion compensation: a method for mitigating the effects of chromatic dispersion in fiber-optic communication links with electronic components in the receiver

emissivity: a measure for how strongly an object emits and absorbs thermal radiation

end pumping: a technique of optically pumping a laser medium in a direction along the laser beam

energy transfer: the phenomenon that dopant ions in laser-active media can exchange excitation energy among each other

enhancement cavities: optical cavities for exploiting the resonant enhancement of the power of circulating light

entrance and exit pupil: the apparent apertures of an optical system, seen from the object or image side

erbium-doped fiber amplifiers: fiber amplifiers based on erbium-doped optical fibers

erbium-doped gain media: laser gain media doped with erbium ions

etalons: monolithic interferometric devices containing two parallel reflecting surfaces

excimer lasers: lasers where optical amplification occurs in a plasma containing excited dimers (or other molecules) with an anti-binding electronic ground state

excited-state absorption: absorption of light by ions or atoms in an excited electronic state, rather than in the electronic ground state

external-cavity diode lasers: non-monolithic diode lasers where the laser cavity (resonator) is completed with external optical elements

eye protection: the protection of eyes against laser hazards

eye-safe lasers: lasers emitting in a wavelength region with relatively low hazards for the human eye


f-number: a measure for the open aperture of a photographic objective

Fabry–Pérot interferometers: interferometers consisting of two highly reflecting mirrors, forming a standing-wave resonator

Faraday isolators: optical isolators based on the Faraday effect

Faraday rotators: devices which can rotate the polarization state of light, exploiting the Faraday effect

femtosecond lasers: lasers emitting pulses with durations between a few femtoseconds and hundreds of femtoseconds

few-mode fibers: fibers supporting only few guided modes

fiber amplifiers: optical amplifiers with doped fibers as gain media

fiber Bragg gratings: reflective structures in the core of an optical fiber with a periodic or aperiodic perturbation of the effective refractive index

fiber bundles: bundles of optical fibers

fiber cables: cables containing one or several optical fibers

fiber cladding: the area(s) around the core of an optical fiber

fiber cleavers: instruments for preparing fiber ends with clean optical surfaces

fiber collimators: devices for collimating the light coming from a fiber, or for launching collimated light into the fiber

fiber connectors: connectors used as terminations of optical fiber cables

fiber core: the region in an optical fiber which guides light

fiber-coupled diode lasers: diode laser devices where the generated light is coupled into an optical fiber

fiber couplers: fiber devices for coupling light from one or several input fibers to one or several output fibers, or from free space into a fiber

fiber fabrication: the fabrication of optical fibers

fiber fuse: the effect that an optical fiber "burns" from the output end under the influence of an intense laser beam launched into the other end

fiber joints: permanent or removable connections between fiber ends

fiber lasers: lasers with a doped fiber as gain medium, or (sometimes) just lasers where most of the laser resonator is made of fibers

fiber launch systems: optical systems for launching light into optical fibers

fiber loop mirrors: fiber loops which act as mirrors

fiber-optic networks: communication systems consisting of fiber-optic links

fiber-optic plates: transparent plates containing many optical fibers

fiber-optic pump combiners: fiber-optic couplers for sending pump and signal light into an amplifier or laser fiber

fiber-optic sensors: optical sensors based on fiber devices

fiber-optic tapers: fiber-optic plates which are tapered for obtaining some image magnification

fiber optics: optics based on optical fibers

fiber patch cables: a protected and connectorized fiber-optic cable

fiber polarization controllers: devices which allow one to control the state of polarization of light within fibers

fiber simulation software: software for numerical simulations on fiber devices

fiber to the home: the technology of providing data connections to private homes via optical fibers rather than electric cables

fibers: a kind of long and thin optical waveguides which can be bent to some degree

field lenses: lenses in imaging instruments which affect the field of view

field of view: the angular extent of the field which can be observed with an optical instruments or the eye

field stops: apertures which limit the field of view of imaging systems

finesse: the free spectral range divided by the FWHM width of the resonances of an optical resonator

flat-top beams: a light beam with a flat intensity profile

fluence: optical energy per unit area

fluorescence: luminescence arising from irradiation with light

fluorescence microscopy: a technique for acquiring microscopic images using fluorescence which is excited in the sample

fluorescent lamps: lamps which emit fluorescent light, usually generated by irradiation of a phosphor with light from an electric gas discharge

fluoride fibers: optical fibers based on fluoride glasses

fluoride glasses: glasses made from compounds of fluorine e.g. with zirconium, aluminum or indium

focal distance: the distance between an objective and its focal point

focal length: a measure of how strongly an optical system focuses or defocuses light

focal plane arrays: arrays of light detectors placed in the focal plane of an imaging system

focal points and focal planes: points to which parallel input rays are concentrated by an optical system, and the planes going through those points

forbidden transitions: transitions between different energy levels of some atoms or ions for which dipole transitions are suppressed via symmetries

four-level and three-level gain media: laser gain media without/with reabsorption from the lower laser level

four-wave mixing: an interaction of light waves based on a χ(3) nonlinearity

Fourier optics: the description of light propagation based on transverse spatial Fourier transforms

free electron lasers: laser devices where light amplification occurs by interaction with fast electrons in an undulator

free-space optical communications: optical data transmission through free space, usually through air or vacuum

free spectral range: frequency spacing of the axial modes of an optical resonator

frequency combs: optical spectra consisting of equidistant lines

frequency doubling: the phenomenon that an input wave in a nonlinear material can generate a wave with twice the optical frequency

frequency metrology: the field of technology dealing with precise frequency measurements

frequency noise: noise of the instantaneous frequency of an oscillating signal

frequency quadrupling: the phenomenon that an input laser beam generates a beam with four times the optical frequency

frequency-resolved optical gating: a technique for the complete characterization of ultrashort optical pulses

frequency tripling: the phenomenon that an input laser beam generates a beam with three times the optical frequency

Fresnel equations: equations for the amplitude coefficients of transmission and reflection at the interface between two transparent homogeneous media

Fresnel lenses: lenses where the surface profile has annular steps

Fresnel number: a parameter determining the regime of diffraction effects

Fresnel reflections: light reflections at optical interfaces

F├╝chtbauer–Ladenburg equation: an equation used for calculating emission cross sections of laser gain media

fundamental mode locking: mode locking with a single pulse circulating in the laser resonator

fusion splicing of fibers: a technique of making low-loss fiber joints by fusing fiber ends together


gain: a measure of the strength of optical amplification

gain bandwidth: the width of the optical frequency range in which significant gain is available from an amplifier

gain clamping: a firm limitation of the gain of some amplifier or laser, usually caused by lasing action

gain efficiency: small-signal gain of an optical amplifier per unit pump power or per unit stored energy

gain equalization: a technique for making the gain spectrum of an optical amplifier device flatter over a certain optical frequency range

gain guiding: guiding of light based on spatially inhomogeneous optical gain

gain media: media for laser amplification

gain narrowing: the phenomenon that the bandwidth of light can be reduced during amplification in a medium with a limited gain bandwidth

gain saturation: the phenomenon that the gain of an amplifier is reduced for high input signal powers

gain switching: a technique for generating short optical pulses in a laser by modulating the laser gain

gas lasers: lasers with a gas (or plasma) as gain medium

Gaussian beams: light beams where the electric field profile in a plane perpendicular to the beam axis can be described with a Gaussian function, possibly with an added parabolic phase profile

Gaussian optics: a framework for describing optical phenomena, which is based on geometrical optics and the paraxial approximation

Gaussian pulses: pulses with a temporal intensity profile which has a Gaussian shape

geometrical optics: a concept of optics which is based on the propagation of light rays, ignoring wave properties of light

Giles parameters: spectroscopic data concerning absorption and amplification in an active fiber

Gires–Tournois interferometers: linear optical resonators used for introducing chromatic dispersion

Gordon–Haus jitter: a timing jitter originating from fluctuations of the center frequency

Gouy phase shift: an additional phase shift occurring in the propagation of focused Gaussian beams

graded-index fibers: optical fibers with a continuously varying refractive index in the radial dimension

gradient-index lenses: lenses which utilize a radial variation of refractive index

green lasers: lasers emitting in the green spectral region

group delay: a measure of the time delay experienced by narrow-band light pulses in an optical device

group delay dispersion: the frequency dependency of the group delay, or (quantitatively) the corresponding derivative with respect to angular frequency

group index: the ratio of the vacuum velocity of light to the group velocity in a medium

group velocity: the velocity with which the envelope of a weak narrow-band optical pulse propagates in a medium

group velocity dispersion: the frequency dependence of the group velocity in a medium, or (quantitatively) the derivative of the inverse group velocity with respect to angular frequency

group velocity mismatch: the phenomenon that pulses become temporally separated in a medium due to different group velocities, or (quantitatively) the difference of inverse group velocities

guided waves: waves for which diffraction is suppressed by a waveguide structure


halogen lamps: incandescent lamps with a halogen filling, allowing for higher operation temperatures

harmonic mode locking: mode locking of a laser where multiple pulses are circulating in the laser resonator with equal temporal spacing

Haus Master equation: an analytical equation describing the evolution of ultrashort pulses in a laser resonator

helium–neon lasers: gas lasers based on a helium–neon mixture

Hermite–Gaussian modes: propagation modes or resonator modes which are described with Hermite–Gaussian functions

high harmonic generation: the phenomenon that very high harmonics of an intense input laser beam are generated in a gas

high-power fiber lasers and amplifiers: fiber lasers and fiber amplifiers with high output powers of e.g. > 10  W

high-power lasers: lasers emitting very high optical powers

higher-order modes: transverse modes of a resonator or waveguide, excluding the fundamental (axial) modes

higher-order solitons: optical pulses in a nonlinear and dispersive medium which exhibit periodic oscillations of their temporal and spectral shape

hollow-core fibers: optical fibers with a hole on the fiber axis

holography: methods for recording and reconstructing three-dimensional images

homogeneous broadening: the increase in the linewidth of an atomic transition caused by effects which affect different radiating or absorbing atoms in the same way

homogeneous saturation: saturation of an absorption or gain, where the shape of the absorption or gain spectrum is not modified

hot mirrors: mirrors which can reduce the heat load in an optical system by reflecting infrared radiation

hyper Raman scattering: Raman scattering processes involving two pump photons

hyperfocal distance: the smallest focusing distance of an imaging instruments for which objects at infinity appear with with reasonably sharp focus


illuminance: the incident luminous flux per unit area on a surface

image intensifiers and image converters: instruments which can intensify images acquired under faint light conditions, or convert images to other wavelengths

image planes: planes which are conjugate to an object plane, so that objects are imaged there

image sensors: optoelectronic sensors which can be used for imaging

imaging: mapping objects points to image points; applications involving such methods

imaging with a lens: creation of an optical image using a single lens

incandescent lamps: light sources which produce thermal radiation from an electrically heated filament

index matching fluids: fluids with a refractive index similar to that of other optical materials, typically used for suppressing light reflections

infrared cameras: cameras which work with infrared light

infrared detectors: detectors for infrared light

infrared emitters: devices emitting infrared radiation

infrared light: invisible light with wavelengths roughly between 750  nm and 1  mm

infrared optics: optical elements and systems which are suitable for infrared light

infrared viewers: instruments for observing scenes with infrared light

inhomogeneous broadening: the increase in the linewidth of an atomic transition caused by effects which act differently on different radiating or absorbing atoms

inhomogeneous saturation: gain saturation effects which alter the shape of the gain spectrum

injection locking: a technique for enforcing operation of a laser on a certain optical frequency by injecting light with that frequency into the laser resonator

injection seeding: a technique for enforcing narrowband operation of a laser or optical parametric oscillator by injecting light into its resonator

insertion loss: power losses due to insertion of a device

instantaneous frequency: temporal derivative of the oscillation phase divided by 2π

integrated optics: the technology dealing with the construction of photonic integrated circuits

integrating spheres: devices with a diffusely reflecting inner surface, used for certain optical measurements

intensity modulators: devices which can be used to modulate the intensity (power) of a light beam

intensity noise: noise of the optical intensity or power of a laser beam

interference: a range of phenomena associated with the superposition of waves

interferometers: optical devices utilizing the phenomenon of interference

interlocks: safety devices for automatically switching off a laser power or interrupting a laser beam

intermodal dispersion: the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode

intracavity frequency doubling: frequency doubling with a nonlinear crystal within the laser resonator

intracavity laser absorption spectroscopy: a method of laser spectroscopy, based on the evolution of the optical spectrum of intracavity radiation under the influence of narrowband absorption features

irradiance: the radiant flux (optical power) per unit area



Kelly sidebands: sidebands in the spectrum of certain mode-locked lasers, related to a periodic disturbance of soliton pulses in the laser resonator

Kerr effect: a nonlinear interaction of light in a medium with an instantaneous response, related to the nonlinear electronic polarization

Kerr lens: a lensing effect arising from the Kerr nonlinearity

Kerr lens mode locking: a technique for mode locking a laser, exploiting nonlinear self-focusing

Kramers–Kronig relations: mathematical relations between absorption coefficient and refractive index of transparent media

Kuizenga–Siegman theory: a theory predicting the durations of pulses from actively mode-locked lasers


lamp-pumped lasers: lasers which are pumped with flash lamps or arc lamps, or in rare cases with tungsten halogen lamps

large-core fibers: optical fibers with relatively large fiber core

large mode area fibers: optical fibers with relatively large mode areas and a single transverse mode or only a few modes

laser absorption spectroscopy: methods of absorption spectroscopy using laser light

laser applications: applications involving laser devices

laser beam characterization: methods of assessing detailed properties of laser beams

laser beams: light beams propagating dominantly in one direction

laser cooling: a variety of techniques for reducing the temperature (i.e., the random motion) of small particles such as atoms or ions

laser cooling units: devices used for cooling of high-power lasers

laser crystals: transparent crystals with laser-active dopants, used as laser gain media

laser design: the design of a laser device, or the process of working out such a design

laser development: the process of developing a laser device

laser diode collimators: devices for collimating the output of laser diodes

laser diode drivers: electronic current supplies for one or several laser diodes

laser diode modules: modules containing diode lasers, and possibly also some optics, cooling devices, electrical elements, etc.

laser diodes: semiconductor lasers with a current-carrying p–n junction as the gain medium

laser dynamics: the temporal evolution of quantities such as the optical power and gain in a laser

laser guide stars: small bright spots in the sky, generated with laser beams for use in astronomy with adaptive optics imaging

laser heads: assemblies containing a mounted gain medium and means for pumping and cooling, or the complete optical parts of a laser, or assemblies for directing a laser beam to a workpiece

laser-induced breakdown: a kind of spark, initiated by intense laser light

laser-induced damage: damage of optical components induced by laser radiation

laser light: light generated with a laser device

laser line optics: optical elements made specifically for certain laser lines

laser lines: narrow spectral lines obtained from lasers

laser marking: a group of methods for labeling materials with lasers

laser microscopy: a technique for generating microscopic images by scanning objects with a laser

laser mirrors: high-quality mirrors used in laser resonators and other optical setups

laser modeling: the investigation of phenomena and quantitative relations in lasers, using theoretical models and computational methods

laser noise: fluctuations of various parameters of laser light, such as the optical power and phase

laser optics: optical elements and systems used with lasers - either in lasers or for transmitting and manipulating laser light

laser physics: the field of physics dealing with phenomena in lasers

laser pointers: devices used for pointing at items with laser beams

laser rangefinders: devices for measuring distances to objects, using lasers

laser resonators: optical resonators serving as basic building blocks of lasers

laser safety: safety of the use of laser devices

laser specifications: descriptions of essential properties of a laser system, such as performance parameters, dimensions, and operating conditions

laser speckle: light patterns resulting from the reflection of coherent light at rough surfaces

laser spectroscopy: methods where the interaction of laser light with matter is utilized

laser threshold: an operation condition of a laser where laser emission just starts to occur

laser transitions: optical transitions where stimulated emission is used to obtain optical amplification

laser viewing cards: cards used for visualizing laser beams in non-visible spectral regions

lasers: devices generating visible or invisible light, based on stimulated emission of light

lasing without inversion: the quantum-optical phenomenon of laser emission occurring without the presence of a population inversion

leaky modes: a concept for quasi-bound waveguide modes with propagation losses due to leakage into the cladding

lenses: transparent optical devices affecting the wavefront curvature of light

LIDAR: a technique for acquiring 3D images with laser pulses

light-emitting diodes: semiconductor diodes that emit light via electroluminescence

light forces: forces associated with absorption, reflection or scattering of light

linewidth: width of the spectrum of a light beam or an absorption feature

linewidth enhancement factor: a parameter quantifying the amplitude–phase coupling in a laser

loupes: optical lenses held close to the eye which are used for viewing objects with some moderate magnification

lower-state lifetime: the lifetime of the population of the lower laser level in the absence of radiation

LP modes: linearly polarized modes of optical fibers with radially symmetric index profiles in the approximation of weak guidance

luminescence: light emission which is not caused by heating

luminous efficacy and efficiency: the luminous flux of a light source divided by its the radiant power or its electrical consumption (efficacy); the ratio of reached efficacy to the theoretically possible efficacy (luminous efficiency)

luminous flux: a measure of the perceived power of light, e.g. from a light source

Lyot filters: optical filter devices based on birefringence, exhibiting a wavelength-dependent transmission


M2 factor: a parameter for quantifying the beam quality of laser beams

magnification: the enlargements of images

magnifying glasses: optical lenses held above objects for viewing them with some moderate magnification

master laser: a single-frequency laser used for injection locking one or several other lasers

master oscillator fiber amplifier: a laser system containing a fiber amplifier for boosting the output power

master oscillator power amplifier: a laser system consisting of a seed laser and a laser amplifier for boosting the output power

McCumber theory: a theory applied to absorption and emission properties of laser gain media, in particular to solid-state media

medical lasers: laser systems used for medical applications

metal–semiconductor–metal photodetectors: fast photodetector devices based on metal–semiconductor (Schottky) contacts

metal-coated mirrors: mirrors where the reflection is obtained from a metal coating

metastable states: excited states (particularly electronic states in laser gain media) which have a relatively long lifetime due to slow radiative and non-radiative decay

Michelson interferometers: an interferometer with two separate arms and a single beam splitter

microbends of fibers: microscopic bends of optical fibers

microchannel plates: electron amplifiers with many spatial channels

microchip lasers: solid-state lasers with a quasi-monolithic setup

microlenses: optical lenses of particularly small diameter, e.g. below 1 mm

microscope objectives: the optical component of a microscope which is next to the observed object

microscopes: instruments for seeing very small objects

mid-infrared fibers: optical fibers for use with mid-infrared light

mid-infrared laser sources: light sources emitting mid-infrared radiation in the form of a laser-like beam

mirror substrates: substrates on which coatings for laser mirrors can be deposited

mirrors: devices which reflect light

mode cleaner cavities: optical cavities used as filters for improving the beam quality of laser beams

mode cleaners: devices which can improve the beam quality of laser beams

mode competition: the phenomenon that different resonator modes experience laser amplification in the same gain medium, leading to cross-saturation effects

mode coupling: a concept for describing and calculating light propagation in certain situations, e.g. involving nonlinear interactions

mode division multiplexing: a multiplexing technique for data transmission in fibers, where different fiber modes are used for different channels

mode field converters: optical devices which allow for efficient coupling between modes of different sizes

mode hopping: the phenomenon that a laser exhibits sudden jumps of optical frequency, which are associated with transitions between different modes of its resonator

mode-locked diode lasers: diode lasers which are actively, passively or hybrid mode-locked for generating ultrashort pulses

mode-locked fiber lasers: ultrafast fiber lasers which are actively or passively mode-locked for generating ultrashort pulses

mode-locked lasers: lasers which emit ultrashort pulses on the basis of the technique of mode locking

mode locking: a group of techniques for generating ultrashort pulses in lasers

mode locking devices: devices for achieving mode locking of a laser

mode matching: the precise spatial matching of the electric field distributions of laser beams and resonator modes or waveguide modes

mode radius: a measure of the transverse extent of a laser mode or laser beam

modes: self-consistent electric field distributions in waveguides, optical resonators or in free space

modes of laser operation: a mode of operation such as continuous-wave operation, Q-switched or mode-locked operation

modulation depth: a relative modulation amplitude, or (for a saturable absorber) the maximum change in absorption

molecular lasers: gas lasers where the laser-active gas consists of molecules rather than separate atoms or ions

monochromatic light: light with a single optical frequency

monochromators: optical devices which transmit only light in an adjustable small range of optical wavelengths

monolithic solid-state lasers: solid-state lasers where the whole laser resonator consists only of one piece of crystal or glass

multi-core fibers: optical fibers containing more than one fiber core

multi-phonon transitions: transitions between electronic levels of atoms or ions in solid media, involving the emission of multiple phonons

multimode fibers: fibers supporting more than one guided mode per polarization direction

multipass amplifiers: an amplifier with multiple passes through the gain medium

multipass gas cells: a cell which allows to realize multiple passes of a laser beam through a gas

multiphonon absorption: absorption processes involving multiple phonons

multiphoton absorption: absorption processes where multiple photons are absorbed simultaneously


nanofibers: optical fibers with transverse dimensions below one micrometer

nanosecond lasers: lasers emitting optical pulses with nanosecond durations

narrow-linewidth lasers: single-frequency lasers with a narrow optical emission spectrum

near field and far field: the optical field either close to a source or object or far from it

neodymium-doped gain media: laser gain media containing laser-active neodymium ions

neutral density filters: optical attenuators with an approximately constant attenuation in a substantial wavelength range

night vision devices: instruments which allow viewing or imaging under conditions of very low light levels

nodal points: points of an optical system for which an incoming ray, directed at a nodal point, leaves the system with the same direction.

noise eaters: devices for reducing the intensity noise of an optical beam by automatically adjusting the degree of power attenuation

noise-equivalent power: the input power to a detector which produces the same signal output power as the internal noise of the device

noise figure: a measure of the excess noise added in an amplifier

noise specifications: specifications for the noise properties of lasers, for example

non-radiative transitions: transitions between energy levels of atoms or ions which are not associated with the emission of light

nonclassical light: light with properties which can be explained only within quantum optics

noncritical phase matching: a technique of phase matching which does not require a critical angular adjustment

nonlinear crystal materials: crystal materials exhibiting an optical nonlinearity, usually of χ(2) type

nonlinear frequency conversion: the conversion of input light to light of other frequencies, using optical nonlinearities

nonlinear index: a parameter for quantifying the Kerr nonlinearity of a medium

nonlinear optics: the part of optics dealing with optical nonlinearities and their applications

nonlinear polarization: the part of the light-induced electric polarization which depends nonlinearly on the electric field of the light

nonlinear polarization rotation: the phenomenon that the change in the polarization direction of light in a fiber can depend on the optical intensity

nonlinear pulse distortion: distortion of the spatial, temporal or spectral characteristics of an optical pulse, resulting from optical nonlinearities

nonlinearities: optical phenomena involving a nonlinear response to a driving light field

nonplanar ring oscillators: a type of monolithic single-frequency lasers, based on a nonplanar ring resonator

numerical aperture: sine of the maximum angle of an incident beam of some optical device, or the sine of the acceptance angle of a waveguide or fiber


object and image space: mathematical spaces for the description of optical elements and systems

objectives: optical elements for collecting light from inspected objects, or for sending light to objects

ocular lenses: lenses or lens systems as used in telescopes and microscopes, for example, close to the observing eye

optical aberrations: degradations of optical images caused in imaging instruments

optical apertures: structures which limit the propagation of light

optical attenuators: devices which can reduce the optical power e.g. of a light beam

optical choppers: devices for periodically interrupting light beams

optical clocks: time measurement devices based on optical frequency standards

optical clockworks: devices which can phase-coherently relate optical frequencies to microwave frequencies

optical coherence tomography: an optical imaging technique where longitudinal resolution is achieved based on interference effects with low-coherence light

optical data transmission: the transmission of information using light beams, e.g. in fibers

optical delay lines: an optical setup which is used to introduce a time delay for a light beam

optical density: a logarithmic measure of the power attenuation, or the refractive index

optical domes: strongly curved optical windows

optical energy meters: instruments for measuring optical pulse energies

optical fabrication: the fabrication of optical components like lenses, prisms and mirrors

optical fiber communications: the technology of transmitting information through optical fibers

optical filters: devices with a wavelength-dependent transmission or reflectance

optical frequency: the oscillation frequency of the electric field of light

optical frequency standards: frequency standards using optical transitions in atoms, ions or molecules

optical glasses: glasses of high quality, made for optical applications

optical heterodyne detection: a sensitive method of photodetection, reaching the standard quantum limit even for weak signal powers

optical intensity: optical power per unit area

optical materials: materials used for optical components

optical metrology: the science and technology of performing measurements with light

optical modulators: devices allowing one to manipulate properties of light beams, such as the optical power or phase

optical molasses: an arrangement of laser beams used for cooling atoms or ions

optical parametric amplifiers: optical amplifiers based on parametric nonlinear interactions

optical parametric chirped-pulse amplification: parametric amplification of chirped ultrashort pulses

optical parametric generators: light sources based on parametric amplifiers with no signal or idler input

optical parametric oscillators: coherent light sources based on parametric amplification within an optical resonator

optical phase: the phase of the electric field of an optical wave, usually considered in the time domain

optical power: the energy per unit time, e.g. transported by a laser beam, or a focusing power

optical power meters: instruments for optical power measurements, based e.g. on heating of an absorber structure or a photodiode

optical power monitors: devices for monitoring optical powers in free-space light beams or in optical fibers

optical profilometers: optical non-contact instruments for precisely measuring surface profiles

optical pumping: electronically exciting a medium with light, or specifically populating certain electronic levels

optical rectification: a nonlinear process where an optical field generates a quasi-DC nonlinear polarization

optical refrigeration: a technique for cooling macroscopic samples via laser-induced fluorescence

optical resonators: arrangements of optical components which allow a beam of light to circulate

optical sampling: a technique of sampling signals using laser light, often in the form of ultrashort pulses

optical slits: narrow openings through which light can be transmitted

optical spectrum: the decomposition of the power or energy of light according to different wavelengths or optical frequencies

optical spectrum analyzers: instruments for measuring optical spectra

optical strain sensors: sensors for mechanical strain based on optical technology, e.g. with fiber Bragg gratings

optical temperature sensors: temperature sensors based on optical technology, e.g. with fiber Bragg gratings

optical thickness: a measure either for the absorption or the effective optical path length of a sample

optical tweezers: arrangements for capturing and moving particles with laser beams

optical windows: flat transparent plates with optical quality, used for protection against the environment

optics: the science and technology dealing with the properties and the propagation of light

opto-isolators: optoelectronic devices for signal transmission with electrical isolation between two electrical circuits

optoelectronics: the technology of electronic devices that interact with light

orientation-patterned semiconductors: semiconductors containing a periodic pattern of domain orientation

oscillators: a device producing an oscillation – in contrast to an amplifier, for example

output couplers: partially transparent laser mirrors, used for extracting output beams from laser resonators

output coupling efficiency: a factor influencing the power efficiency of a laser, taking into account intracavity losses


p–i–n photodiodes: photodiodes with a p–i–n semiconductor structure

parabolic mirrors: mirrors with the surface shape of a paraboloid, or at least a parabolic cross section

parabolic pulses: pulses with a parabolic intensity profile

parametric amplification: a process of optical amplification based on a parametric nonlinearity

parametric fluorescence: emission of light by a parametric amplifier without signal or input inputs

parametric nonlinearities: instantaneous optical nonlinearities based on the χ(2) or χ(3) nonlinear tensor of a transparent medium

parasitic lasing: unwanted laser operation in a laser or amplifier device

paraxial approximation: a frequently used approximation, essentially assuming small angular deviations of the propagation directions from some beam axis

passive mode locking: a technique of mode locking, based on a saturable absorber inside the laser resonator

peak power: maximum optical power of a pulse

periodic poling: a technique for achieving quasi-phase matching of nonlinear interactions in a transparent crystal material

phase matching: a group of techniques for achieving efficient nonlinear interactions in a medium

phase-matching bandwidth: the width of an optical frequency range in which some process (e.g. a nonlinear interaction) can be efficient due to more or less precise phase matching

phase modulators: devices for manipulating the phase of a laser beam

phase noise: noise of the optical phase of a beam or of an electric signal

phase shift method for distance measurements: a method for optical distance measurements

phase velocity: the velocity with which phase fronts propagate in a medium

phonons: quantized microscopic vibrations in solid media

phosphate glasses: certain glasses from which certain optical fibers and laser gain media can be made, for example

phosphorescence: a luminescence which lasts for a relatively long time after excitation of a medium

phosphors: substances which exhibit luminescence, used mostly for illumination and displays

photo cameras: optical instruments for taking photographic images

photocathodes: an electrode where electrons can be released based on the photoelectric effect

photochromic materials: optical materials which exhibit increased light absorption when being exposed to light

photoconductive detectors: photodetectors which exhibit a reduction in electrical resistance caused by incoming light

photoconductive sampling: a technique of optical sampling with photoconductive switches

photoconductive switches: electric switches controlled by light via photo-induced conductivity

photocurrent: an electric current which is induced by incident light in a photodetector

photodarkening: the phenomenon that the optical power losses in a medium can grow when the medium is irradiated with light at certain wavelengths

photodetectors: devices used for the detection of light

photodiode arrays: typically linear arrays of photodiodes, provided as compact devices

photodiodes: semiconductor devices with a p–n or p–i–n structure for the detection of light

photoelectric effect: the emission or excitation of electric carriers related to the absorption of light

photoemissive detectors: photodetectors which are based on the external photoelectric effect

photographic objectives: objective lenses used for photography

photoluminescence: emission of light which is caused by the irradiation of a material with other light

photometers: instruments for measuring light properties such as irradiance or illuminance, or optical properties of objects

photometry: the science and technology of measuring properties of light related to its perceived brightness for human eyes

photomultipliers: photodetection devices based on the photoelectric effect and charge multiplication by secondary emission of electrons

photon counting: photodetection at low light levels where single photon absorption events are counted

photonic bandgap fibers: optical fibers where light is guided based on a photonic bandgap effect

photonic crystal fibers: specialty optical fibers with a built-in microstructure, in most cases consisting of small air holes in glass

photonic integrated circuits: integrated circuits with optical functions

photonic metamaterials: nanostructured materials with special optical properties

photonics: the science and technology of light

photons: quanta of light energy

phototransistors: a type of semiconductor-based photodetector with internal current amplification for higher responsivity

phototubes: gas-filled or vacuum tubes which can be used as photodetectors

picosecond lasers: lasers emitting pulses with picosecond durations

pinholes: circular holes, typically with diameters well below one millimeter

planar waveguides: waveguide structures guiding light only in one dimension

plane waves: waves with plane wavefronts

plastic optical fibers: optical fibers made of polymer materials

plastic optics: optical elements made of organic polymer materials

Pockels cell drivers: electronic devices for driving Pockels cells

Pockels cells: electro-optic devices, used for building modulators

Pockels effect: the phenomenon that the refractive index of a medium exhibits a modification which is proportional to the strength of an applied electric field (linear electro-optic effect)

polarization beam combining: a technique for combining two polarized laser beams

polarization beat length: the period of interference effects in a birefringent medium

polarization-maintaining fibers: specialty optical fibers with strong built-in birefringence, preserving the properly oriented linear polarization of an input beam

polarization mode dispersion: the polarization dependence of the propagation characteristics of light waves in optical fibers

polarization of light: direction of the electric field oscillation of a light beam

polarization waves: patterns of electric (or magnetic) polarization of a medium which are usually caused by light waves

polarizers: optical filters where the light transmission depends strongly on the direction of polarization

polychromatic light: light with multiple optical frequencies

population inversion: a state of a medium where a higher-lying electronic level has a higher population than a lower-lying level

position-sensitive detectors: photodetectors which can detect the position of a light spot in one or two dimensions

power density: intensity or power per unit frequency interval

power over fiber: delivery of power for electronic devices via light in an optical fiber

power scaling of lasers: a procedure for substantially increasing the output power of lasers

power spectral density: optical power or noise power per unit frequency interval

principal dispersion: a crude measure for chromatic dispersion of a transparent optical material

principal points and principal planes: planes from which rays appear to be deflected in an optical system according to paraxial optics, and the intersections of those planes with the optical axis

prism pairs: combinations of two prisms, mostly used for dispersion compensation

prisms: transparent optical devices which refract or reflect light

propagation constant: a mode- and frequency-dependent quantity describing the propagation of light in a medium or waveguide

pulse characterization: the measurement of various properties of an optical pulse

pulse compression: linear or nonlinear techniques for reducing the durations of optical pulses

pulse duration: the duration of an optical pulse

pulse energy: energy of an optical pulse

pulse front tilt: the phenomenon that the arrival time of an ultrashort pulse varies across the beam profile

pulse generation: the generation of light pulses (flashes)

pulse pickers: electrically controlled optical switches used for extracting single pulses from a pulse train

pulse propagation modeling: working with physical models describing the propagation of ultrashort pulses e.g. in lasers or optical fibers

pulse repetition rate: the number of pulses emitted per second e.g. by a mode-locked or Q-switched laser

pulse stretchers: devices for temporally stretching ultrashort light pulses

pulsed laser deposition: a method for fabricating thin films on substrates by evaporating materials with laser pulses

pulsed lasers: lasers emitting light in the form of pulses

pulses: flashes of light

pump–probe measurements: techniques for investigating ultrafast phenomena, where a pump pulse excites a sample and a probe pulse is used for probing the sample after an adjustable delay time

pump absorption: the absorption of pump light, e.g. in a laser or a laser amplifier

pump chambers: a part of a lamp-pumped solid-state laser, containing a laser rod, one or more lamps, reflectors, a cooling arrangement and possibly other parts

pump depletion: reduction in the transmitted pump power in a laser or nonlinear conversion device

pump parameter: the ratio of pump power and threshold pump power of a laser

pyroelectric detectors: light detectors based on the pyroelectric effect


Q factor: a measure of the damping of resonator modes

Q-switched lasers: lasers which emit optical pulses, relying on the method of Q switching

Q-switched mode locking: an operation regime of mode-locked lasers with strong fluctuations of the pulse energy

Q switches: optical switches which are typically used for generating nanosecond pulses in lasers

Q switching: a method for obtaining energetic pulses from lasers by modulating the intracavity losses

Q-switching instabilities: instabilities of passively mode-locked lasers where the pulse energy is unstable

quantum cascade lasers: semiconductor lasers relying on intersubband transitions, normally emitting in the mid-infrared spectral region

quantum cryptography: methods to encrypt information securely, relying on quantum-mechanical phenomena

quantum defect: energy difference between pump and laser photons

quantum dots: microscopic structures confining charge carriers in three dimensions

quantum efficiency: percentage of input photons which contribute to a desired effect

quantum electronics: an area of physics which is the basis of optoelectronics

quantum key distribution: methods for the secure distribution of encryption keys

quantum noise: noise arising from quantum fluctuations

quantum optics: the part of optics which deals with quantum effects

quantum wells: thin layers confining carriers in one dimension

quarter-wave mirrors: dielectric mirrors based on a sequence of quarter-wave layers

quasi-continuous-wave operation: an operation mode of lasers (e.g. diode bars) where the pump power is switched on for short time intervals in order to limit thermal effects

quasi-phase matching: a technique of achieving a kind of phase matching, using a periodic structure

quasi-soliton pulses: soliton-like pulses in lasers or fiber-optic links

quenching: the reduction or limitation of an excited-state population, mostly by unwanted effects


Rabi oscillations: oscillations of level populations (or quantum mechanical probability amplitudes) under the influence of an incident light field

radiance: the optical power per unit area and solid angle

radiant energy: energy of some electromagnetic radiation which is emitted, transmitted or absorbed during some interval of time

radiant flux: radiant energy per unit time which is emitted, transmitted, reflected or received by an object

radiation-balanced lasers: lasers where the heat generation in the gain medium is compensated by optical refrigeration in the same medium

radiation pressure: a mechanical pressure resulting from incident light or other radiation

radiative lifetime: lifetime of an electronic state in the (hypothetical) situation where only radiative processes depopulate that level

radiometry: the science and technology of measuring properties of electromagnetic radiation, including light

Raman amplifiers: optical amplifiers based on Raman gain

Raman gain: optical gain arising from stimulated Raman scattering

Raman lasers: lasers based on Raman gain rather than on laser gain from stimulated emission

Raman scattering: a nonlinear scattering process involving optical phonons

Raman spectroscopy: optical spectroscopy based on spontaneous or stimulated Raman scattering

rare-earth-doped fibers: optical glass fibers which are doped with rare earth ions

rare-earth-doped gain media: laser gain media which are doped with rare earth ions

rate equation modeling: a method of modeling the dynamics of laser gain media

Rayleigh length: the distance from a beam waist where the mode radius increased by a factor square root of 2

Rayleigh scattering: scattering of light at scattering centers which are much smaller than the wavelength

reciprocity method: a spectroscopic method which is often used for obtaining the scaling of emission spectra of laser gain media

recirculating fiber loops: fiber loops in which light can circulate many times

recoil limit: a lower limit for the temperature which can be reached with some laser cooling techniques

red lasers: lasers emitting red light

reference cavities: optical cavities serving as a kind of frequency reference

reflectance: the ratio of reflected optical power to the incident optical power at some reflecting object

reflectivity: the ratio of reflected optical power to the incident optical power at a surface

refraction: the change of the propagation direction when a wave comes from one medium into another one

refractive index: a measure of the reduction in the velocity of light in a medium

regenerative amplifiers: optical amplifiers with a resonator in which a light pulse can do multiple round trips before being coupled out

relative intensity noise: noise of the optical intensity (or actually power), normalized to its average value

relaxation oscillations: small mutually coupled oscillations of the laser power and laser gain around their steady-state values

resonant frequency doubling: frequency doubling with a nonlinear crystal placed in a resonant enhancement cavity

resonator design: the description of the optical components from which an optical resonator is made, and their exact arrangement

resonator modes: modes of an optical or microwave resonator

responsivity: photocurrent per unit optical power incident on a photodetector

retroreflectors: optical devices which reflect light in a direction which is parallel to the incident beam

return loss: a measure for how much reflected light is attenuated

RGB sources: source of red, green and blue light, which is usually provided in the form laser beams

ring lasers: lasers with a ring resonator

ring resonators: optical resonators where light can independently circulate in two different directions

rod lasers: solid-state lasers with a rod-shaped gain medium

ruby lasers: solid-state lasers based on a ruby crystal

rugate filters: dielectric interference filters with a continuous variation of the refractive index


saturable absorbers: light absorbers with a degree of absorption which is reduced at high optical intensities

saturable Bragg reflectors: a special type of saturable semiconductor absorbers

saturation energy: a measure of the incident optical pulse energy required for achieving significant saturation of an absorber or a gain medium

saturation power: a measure of the incident optical power required for achieving significant saturation of an absorber or a gain medium

scanning lenses: lens systems which are designed for use in laser scanners

scattering: processes where light is sent in other directions, usually but not always in random directions

Schawlow–Townes linewidth: linewidth of a single-frequency laser with quantum noise only

sech2-shaped pulses: pulses with a temporal intensity profile which has the shape of a sech2 function

seed lasers: lasers which are used for generating some seed light into an amplifier or another laser

self-focusing: focusing of a beam in a transparent medium, caused by the beam itself through a nonlinear process in the medium

self-heterodyne linewidth measurement: a technique for laser linewidth measurements, based on a beat note between the beam and a delayed version of itself

self-phase modulation: nonlinear phase modulation of a beam, caused by its own intensity via the Kerr effect

self-starting mode locking: mode locking which is quickly achieved after turning on a laser, without external intervention

self-terminating laser transitions: laser transitions which can not be continuously operated due to the accumulation of population in the lower laser level

Sellmeier formula: an equation for calculating the wavelength-dependent refractive index of a medium

semiconductor lasers: lasers based on semiconductor gain media

semiconductor optical amplifiers: optical amplifiers based on semiconductor gain media

semiconductor saturable absorber mirrors: saturable semiconductor absorber devices acting as nonlinear mirrors

sensitivity: (different meanings, depending on the context)

shot noise: quantum-limited intensity noise

side pumping: a technique of pumping a solid-state laser in directions which are approximately transverse to its beam direction

signal-to-noise ratio: the ratio of signal power to noise power in a detector

silica fibers: optical fibers based on fused silica or related materials

silicon photonics: photonic technology based on silicon chips

single-atom lasers: lasers with only a single atom as the gain medium

single-crystal fibers: long and thin rods made of monocrystalline materials

single-frequency lasers: lasers emitting radiation in a single resonator mode

single-frequency operation: an operation mode of a laser where only a single resonator mode acquires a significant power

single-mode fibers: optical fibers supporting only a single guided mode per polarization direction

single-mode operation: operation of a laser on axial (longitudinal) resonator modes only, or even on a single axial mode

single-polarization fibers: optical fibers which transmit only light with a certain polarization

Sisyphus cooling: a mechanism for laser cooling of atoms or ions

slab lasers: solid-state bulk lasers where the gain medium has the form of a slab

slave laser: a laser which is injection-locked to another laser

slope efficiency: differential power efficiency of a laser

small-signal gain: optical gain for small signal intensities, where no amplifier saturation occurs

solid-state lasers: lasers based on solid-state gain media (usually ion-doped crystals or glasses)

soliton mode locking: a mechanism for laser mode locking based on soliton pulses

soliton period: the distance over which higher-order solitons reproduce their temporal and spectral shape

solitons: pulses with a certain balance of nonlinear and dispersive effects

space division multiplexing: a multiplexing technique for optical data transmission where multiple spatial channels are utilized

spatial hole burning: a distortion of the gain shape in a laser medium (or the loss spectrum in a saturable absorber medium), caused by saturation effects of a standing wave

spatial walk-off: the phenomenon that the intensity distribution of a beam in an anisotropic crystal drifts away from the direction of the wave vector

specialty fibers: optical fibers with special designs or materials

spectral beam combining: a class of methods for beam combining, based on wavelength-sensitive beam combiners

spectral brightness: brightness per unit optical bandwidth

spectral interferometry: a class of interferometric methods, where interference in the frequency domain is exploited

spectral lines: narrow spectral features related to emission or absorption lines of certain substances

spectral phase: the phase of the electric field in the frequency domain

spectral response of a photodetector: the range of optical wavelengths for which a photodetector is sensitive

spectrograms: a type of diagrams which visualize optical or other signals

spectrographs: instruments for spectrally analyzing light

spectrometers: devices for separating spectral components

spectrophotometers: devices for measuring the wavelength-dependent transmittance or reflectance of solutions or solid objects

spectroscopy: methods where the interaction of light with matter is utilized

spherical aberrations: optical aberrations resulting from spherical optical surfaces

spiking: the tendency of some lasers to emit spikes of output power, particularly after being switched on

spontaneous emission: a quantum effect, causing the spontaneous decay of excited states of atoms or ions

squeezed states of light: nonclassical states of light with noise below the standard quantum limit in one quadrature component

stability zones of optical resonators: parameter regions of an optical resonator where the resonator is geometrically stable

stabilization of lasers: measures applied to lasers in order to improve their stability in terms of output power, optical frequency, or other quantities

standard quantum limit: a limit for noise levels set by quantum mechanics

standard spectral lines: spectral lines which are often used as wavelength references

step-index fibers: optical fibers with a step-index refractive index profile

stimulated emission: a quantum effect, where photon emission is triggered by other photons

Stokes shift: the reduction in photon energy in certain processes

streak cameras: devices for measuring the power of an optical pulse versus time

sum and difference frequency generation: nonlinear processes generating beams with the sum or difference of the frequencies of the input beams

supercontinuum generation: a nonlinear process for strong spectral broadening of light

superfluorescence: collective emission of radiation by an ensemble of excited atoms or ions

superluminal transmission: transport of some quantity with a velocity exceeding that of light in vacuum

superluminescence: fluorescence which is enhanced by laser amplification

superluminescent diodes: broadband semiconductor light sources based on superluminescence

superluminescent sources: optical sources based on superluminescence

supermirrors: laser mirrors with a very high reflectivity

superradiance: collective emission of an ensemble of atoms or ions after coherent excitation

surface-emitting semiconductor lasers: semiconductor lasers where the generated light propagates in the direction perpendicular to the wafer surface

synchronization of lasers: the synchronization of light pulses generated with lasers

synchronous pumping: pumping of a laser or an optical parametric oscillator with short light pulses in synchronism with the emitted pulses


tapered fibers: optical fibers which are over some length stretched out to a very small diameter

telecentric lenses: lenses or objectives having the entrance or exit pupil at infinity

telecom fibers: optical fibers for use in optical fiber communications

telescopes: optical instruments for viewing distant objects, or sometimes more generally afocal systems

temporal walk-off: the loss of temporal overlap of pulses, caused by group velocity mismatch

terahertz radiation: electromagnetic radiation with terahertz frequencies

thermal detectors: light detectors which are based on measuring light-induced temperature rises

thermal lensing: a lensing effect induced by temperature gradients

thermal radiation: electromagnetic radiation which results from thermal emission

thin-disk lasers: solid-state bulk lasers having a very thin disk of laser-active material as the gain medium

thin-film polarizers: optical polarizers based on a multilayer dielectric coating

third-order dispersion: chromatic dispersion related to a third-order dependence of the phase change on the frequency offset

threshold pump power: the pump power at which the laser threshold is reached

thresholdless lasers: lasers with a threshold power which is virtually zero

time–bandwidth product: product of temporal and spectral width of a pulse

time division multiplexing: a multiplexing technique operating in the time domain

time-of-flight measurements: distance measurements based on measuring the time of flight of a light pulse

time-resolved spectroscopy: spectroscopy where time-dependent signals are measured

timing jitter: fluctuations of the temporal positions of pulses

timing phase: a phase reflecting timing deviations by relating them to the pulse period

titanium–sapphire lasers: lasers based on a Ti:sapphire gain medium

total internal reflection: the phenomenon that light incident on the boundary between two transparent media is fully reflected if the light is coming from the optical more dense medium and the angle of incidence is sufficiently large

transform limit: a limit for the time–bandwidth–product of an optical pulse

transition cross sections: material parameters for quantifying the likelihood or rate of optical transition events

transition-metal-doped gain media: laser gain media which are doped with transition metal ions

transmissivity: the ratio of transmitted optical power to the incident optical power at a surface

transmittance: the ratio of transmitted optical power to the incident optical power at some partial transparent object

triangulation: a method for distance measurements

tunable lasers: lasers the output wavelengths of which can be tuned

tunable optical filters: optical filters with tunable filter parameters, e.g. concerning center wavelength of pass band

tungstate lasers: solid-state lasers based on tungstate crystals as gain media

twisted-mode technique: a technique for achieving single-frequency operation of a laser, based on quarter-wave plates on both sides of the gain medium

two-photon absorption: a nonlinear absorption process where two photons are simultaneously absorbed

Twyman–Green interferometers: an interferometer similar to a Michelson interferometer, but with expanded beams


ultrafast amplifiers: amplifiers for ultrashort optical pulses

ultrafast laser physics: the area of laser physics dealing with ultrashort processes

ultrafast lasers: lasers emitting ultrashort pulses

ultrafast optics: the part of optics dealing with ultrafast phenomena

ultrashort pulses: optical pulses with durations of picoseconds or less

ultraviolet lasers: lasers (or other laser-based light sources) generating ultraviolet light

ultraviolet light: invisible light with wavelengths shorter than ≈ 400  nm

ultraviolet optics: optical elements for use with ultraviolet light

unstable resonators: optical resonators which are dynamically unstable with respect to transverse beam offsets

upconversion: a process where light can be emitted with photon energies higher than those of the light generating the excitation

upconversion lasers: lasers relying on upconversion processes

upper-state lifetime: the lifetime of the population of the upper laser level


V number: a normalized frequency parameter, which determines the number of modes of a step-index fiber

vacuum noise: quantum noise of the vacuum state, or quantum noise introduced into an optical system via a vacuum input

vanadate lasers: lasers based on rare-earth-doped yttrium, gadolinium or lutetium vanadate crystals, usually Nd:YVO4

variable optical attenuators: optical attenuators with a variable degree of attenuation

VCSEL arrays: arrays containing many VCSEL laser emitters

velocity-matched photodetectors: a type of photodetector where propagation velocities of light and electrical signals are matched in order to achieve a very high detection bandwidth

velocity of light: the velocity with which light propagates in a medium

vertical cavity surface-emitting lasers: a monolithic kind of semiconductor lasers with beam emission perpendicular to the wafer surface

vertical external-cavity surface-emitting lasers: surface-emitting semiconductor lasers with an external laser resonator

vibronic lasers: lasers based on gain media with a large gain bandwidth, caused by a strong interaction of electronic transitions with phonons

vignetting: a reduction of image brightness in peripheral regions

visible lasers: lasers (or other laser-based light sources) emitting visible light

volume Bragg gratings: Bragg gratings written into some bulk material


wall-plug efficiency: total electrical-to-optical power efficiency of a laser system

wave optics: the description of optical phenomena based on wave models

wave vector: a vector indicating the direction of wave propagation and the phase delay per unit length

waveguide dispersion: chromatic dispersion arising from waveguiding effects

waveguide lasers: lasers with a waveguide structure in the gain medium

waveguides: spatially inhomogeneous transparent structures for guiding light

wavelength: the spatial period of a plane wave

wavelength division multiplexing: a multiplexing technique working in the wavelength domain

wavelength-swept lasers: lasers which allow for period and fast wavelength tuning

wavelength tuning: the manipulation of the output wavelength of an optical device such as a laser or an optical parametric oscillator

wavemeters: devices for precise measurements of laser wavelengths

wavenumber: the phase delay per unit length, or that quantity divided by 2π

waveplates: transparent plates with a defined amount of birefringence, used for modifying the polarization of light

wedge prisms: prisms with a small angle between the end surfaces

white light interferometers: interferometers using broadband light inputs

white light sources: light sources with very broad optical bandwidth

working distance: the distance between an objective and the object


X-ray lasers: lasers emitting in the X-ray spectral region


YAG lasers: lasers based on YAG (yttrium aluminum garnet) crystals, usually Nd:YAG or Yb:YAG

yellow and orange lasers: lasers emitting yellow or orange light

YLF lasers: lasers based on YLF (yttrium lithium fluoride) crystals, usually Nd:YLF

ytterbium-doped gain media: laser gain media containing laser-active ytterbium ions


z-scan measurements: a technique for measuring the strength of the Kerr nonlinearity of a material, relying on self-focusing

zero dispersion wavelength: a wavelength where the group delay dispersion of a fiber or a material is zero

List of Formula Symbols

SymbolExplanationSI Units
a core radius of a fiber m
b confocal parameter of a laser beam (twice the Rayleigh length) m
c velocity of light m/s
deff effective nonlinear coefficient of a nonlinear crystal material m/V
dij components of the χ(2) nonlinear tensor m/V
e elementary electric charge C
f focal length (e.g., of a lens) m
f noise frequency Hz
frep pulse repetition rate Hz
fro relaxation oscillation frequency Hz
g gain coefficient 1 or m−1
g(τ) complex degree of coherence 1
gB nonlinear gain coefficient for Brillouin scattering m/W
gR nonlinear gain coefficient for Raman scattering m/W
gss small-signal gain coefficient 1 or m−1
h Planck's constant J s
k wavenumber m−1
kB Boltzmann constant J/K
Δk phase mismatch of a parametric nonlinear process rad/m
lrt round-trip losses of a resonator 1
n refractive index 1
ne extraordinary refractive index 1
neff effective refractive index 1
ng group index 1
no ordinary refractive index 1
n2 nonlinear index m2/W
q complex q parameter (→ Gaussian beams) m
r radius (e.g., in fiber core or distance from beam axis) m
r pump parameter of a laser 1
rij components of the electro-optic tensor m/V
vg group velocity m/s
vp phase velocity m/s
w Gaussian beam radius m
w0 beam radius at the beam waist 1
zR Rayleigh length m
zs soliton period m
A amplitude of pulse envelope
Aeff effective mode area m2
B B integral rad
B noise bandwidth Hz
B brightness W/(m2 rad2)
BPP beam parameter product mm mrad
Dλ dispersion parameter ps/(nm km)
E electric field strength V/m
Ep pulse energy J
Esat saturation energy J
F finesse 1
F noise figure 1
Fsat saturation fluence J/m2
GX(τ) autocorrelation function of some variable X
I optical intensity W/m2
Isat saturation intensity W/m2
Lcoh coherence length m
M2 M2 factor 1
NF Fresnel number 1
Ndop doping concentration m−3
NA numerical aperture 1
P optical power W
P electric polarization of a medium C/m2
Pout output power W
Pp peak power of a pulse W
Psat saturation power of a gain medium or saturable absorber W
Q Q factor 1
R power reflectivity (or reflectance) (of a mirror) 1
R radius of curvature (e.g., of wavefronts) m
R responsivity of a photodiode A/W
ΔR modulation depth of a saturable absorber 1
Sφ(f) power spectral density of a phase value rad2/Hz
T temperature K or °C
Tg group delay s
Toc output coupler transmission 1
Trt round-trip time of an optical resonator s
V V number of a step-index fiber 1
α absorption coefficient m−1
α linewidth enhancement factor 1
α thermal expansion coefficient K−1
β propagation constant rad/m
β2 group velocity dispersion s2/m
γSPM self-phase modulation coefficient rad/(W m)
ε0 electric permittivity of vacuum C/Vm
ε relative electric permittivity 1
η efficiency 1
ηoc output coupling efficiency 1
θ beam divergence half-angle rad
θ polar angle rad
κ thermal conductivity W/(m K)
λ optical wavelength m
λp pump wavelength m
λs signal wavelength m
μ relative magnetic permeability 1
μ0 magnetic permeability of vacuum N/A2
ν optical frequency Hz
νB Brillouin frequency shift Hz
νceo carrier–envelope offset frequency Hz
ν(t) instantaneous frequency Hz
Δν optical bandwidth Hz
ρ walk-off angle rad
σabs absorption cross section m2
σem emission cross section m2
τ2 upper-state lifetime s
τcoh coherence time s
τp pulse duration s
τrad radiative lifetime s
φ azimuth angle rad
φ optical phase or phase shift rad
φG Gouy phase shift rad
φnl nonlinear phase shift from the Kerr effect rad
φt timing phase rad
Δφceo carrier–envelope offset phase rad
χ(2), χ(3) nonlinear tensor 1
ω angular frequency rad/s, s−1
Λ modulation period of a fiber Bragg grating m

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